Many of the polycyclic aromatic hydrocarbons are carcinogenic. The excited states of these molecules are also involved in the irreversible binding to the DNA in U.V. carcinogenesis. Coumarins and furocoumarins also show similar photochemical and photobiological effects. For example, xanthotoxin and psoralen are highly mutagenic and sometimes skin-cancerigenic upon topical applications under the influence of light. Aflatoxins, which contain the coumarin chromophore, are potent carcinogens. Many furocoumarins are responsible for skin erythema and related skin diseases, particularly common in the Southwest U.S. owing to coumarin-producing plants and abundant sun-light. In addition to naturally occurring coumarin compounds including aflatoxins, synthetic analogs possessing the unique photoreactivity for cross-linking the DNA strands, benzodipyrones, will be examined with respect to their photobiological reactivity, carcinogenic anti-tumor potential. The specific objective of the present proposal then is to elucidate the chemical mechanisms of interactions between DNA and skin-sensitizing and -carcinogenic coumaryl compounds under the influence of light. Information concerning the mode of interactions (intercalation, photocycloaddition, and cross-linking) will be obtained, and biological effects of these interactions will be ascertained in terms of specific effects of the photocycloaddition of these compounds to Bacillus subtilis DNA. In addition, effects of the photocycloaddition of coumaryl derivatives to tryptophan residues of enzymes and pyrimidine bases of t-RNA will be investigated in order to assess molecular mechanisms of these compounds in Photodynamic action in general and skin-sensitization and U.V. carcinogenesis in particular.